A ring-shaped synchronous network system according to the related art has a configuration in which two or more communication devices are connected in a dual ring structure and includes a time master station supplying a reference time and time slave stations operating on the basis of the reference time of the time master station. In such a ring-shaped synchronous network system, the stations are synchronized with each other as follows (for example, see Patent Literature 1): First, propagation delay times from the time master station to the time slave stations are measured in advance using a propagation delay measurement frame; and the time slave stations then correct a slave clock counter with a correction value reflecting the propagation delay when a synchronization frame is received from the time master station.
Patent Literature 1 contains a proposal for a configuration for preventing unsuitable synchronization correction when the arrival time of a synchronization frame varies due to communication disturbances or the like, when a synchronization frame arrives at the slaves with a time delay, or when a false synchronization frame due to noise or the like causes contamination. The time slave station corrects the slave clock counter when a synchronization frame is received within a synchronization frame reception-enabled time period, and it does not correct the slave clock counter otherwise. The synchronization frame reception-enabled time period is a time period with a duration calculated on the basis of the fluctuation of the synchronization timing (or synchronization jitter) present in a synchronization system or a duration calculated on the basis of the target synchronization accuracy. The period starts at a timing reflecting the synchronization frame propagation delay times between the time master station and the time slave stations in clockwise and counterclockwise paths. The synchronization frame reception-enabled time period enables synchronization using a synchronization frame (a process of correcting the slave clock counter according to a received synchronization frame) only during this period.
Since the slave clock counter is used to control the synchronization frame reception-enabled time period, a time interval (hereinafter, referred to as a synchronization correction cycle) in which a slave clock is periodically corrected using the synchronization frame and a time interval (hereinafter, referred to as a communication cycle) in which a frame from a specific station is periodically received are equal to each other (for example, see Patent Literature 2).